The classical radiographic methods, such as computer tomography, mammography, angiography, X-ray inspection techniques or comparable methods produce a display of the attenuation of an X-ray beam along its path from the X-ray source to the X-ray detector. The attenuation is caused by the medium or materials that the radiation passes through along the beam path. It is normally indicated or recorded in the form of the attenuation coefficient μ, which is defined as the logarithm of the ratio of the intensity of the attenuated radiation to the primary radiation with respect to a path normal.
Increased attenuation values may be caused either by materials having a relatively high atomic number, such as calcium in the skeleton or iodine in a contrast agent, or by a higher material density, for example in the case of a lung node. The local attenuation coefficient μ at a measurement point is dependent on the X-ray energy injected into the tissue or material located there, on the local tissue or material density ρ, and on the atomic number Z of the material at the measurement point.
The energy-dependent X-ray absorption of a material, as is defined by its effective atomic number, is thus superimposed on the X-ray absorption that is influenced by the material density. Materials and tissues of different chemical or physical composition may thus have identical attenuation values in the X-ray image. Conversely, on the other hand, it is impossible to deduce the material composition of an object being examined from the attenuation value in an X-ray recording.
Correct interpretation of the distribution (which is thus actually rather unclear) of the X-ray attenuation values in an X-ray image produced using a radiographic examination method can generally be carried out only on the basis of morphological criteria in the medical sector, and generally requires a radiologist with decades of experience in his field. Nevertheless, in some circumstances, structures which occur with increased attenuation values in the imaging process for an X-ray examination cannot be clearly classified. For example, it is difficult to distinguish between calcification close to the hilus on a thorax overview recording and a vessel which is located orthogonally with respect to the imaging plane. It is also virtually impossible to distinguish, for example, between diffuse calcification and fresh bleeding.
Even in the case of material and safety examinations, the examiner generally supplements the information in the display of an attenuation value distribution by his personal specialist knowledge and professional experience. Nevertheless, it is impossible, for example, for him to distinguish reliably between plastic-bonded explosive mixtures and a non-explosive plastic directly from an X-ray image.
Methods for displaying material-characteristic values are required for this purpose. One such method is described, for example, in the German Patent Application with the file reference 10143131.7. The method makes use of the fact that, with a defined X-ray spectrum, the X-ray attenuation values for specific value pairs (ρ, Z), namely combinations of the material density ρ and of the atomic number Z, are identical and together form a so-called iso-absorption line in the ρ-Z plane. If a second, different type of X-ray spectrum is used, a second iso-absorption line with a different profile is obtained, which intersects the first at a point whose coordinates in the ρ-Z plane reflect the material density ρ and the atomic number Z of the material through which the beam has passed.
In the context of this description, the expression atomic number is not used in the strict sense relating to the elements. Instead, this denotes an effective atom number of a tissue, or material, which is calculated from the chemical atomic numbers and atomic weights of the elements which are involved in the formation of the tissue or material. The precise equation for determination of an atomic number in the sense described above is quoted in the patent application that has been mentioned.
Furthermore, the expression X-ray spectrum in this context is not restricted to the spectral distribution of X-ray radiation emitted from an X-ray source, but additionally takes into account the different weighting of different spectral areas in the emission spectrum of the X-ray tube at the X-ray detector end. A measured attenuation value is thus obtained from the direct attenuation of the beam spectrum emitted from the X-ray tube and the spectrum efficiency of the X-ray detector is used. Both values are system-specific variables and may vary over the course of time.
The profile of an iso-absorption line in the ρ-Z plane is critically influenced by the respective spectral distribution of an X-ray spectrum. Since the recording of the spectral profile of the X-ray spectrum in an X-ray system by measurement is highly complex, the attenuation values of various calibration materials are determined in an X-ray system with the various X-ray spectra that are used in it in order to avoid the need for corresponding measurements. The measurements are repeated at specific intervals in order to take account of any changes in the X-ray spectra with time. The calibration materials differ from one another in their material densities and, preferably, in their atomic number as well. The measured values form support points for subsequent calculation of iso-absorption lines. The iso-absorption lines are used to calculate a density function ρ(μ1, μ2) and an atomic number function Z(μ1, μ2), which associate a density and an atomic number respectively, with a value pair of attenuation values μ1 and μ2 for a material when using a first and a second X-ray spectrum.
However, in practice, it has been found that the calculation of the density and atomic number functions is highly inaccurate. Acceptable results are achieved only with calibration samples with medium-range atomic numbers. Density and atomic number functions obtained from corresponding calibration measurements are thus highly unreliable in the area both of small and large atomic numbers.
However, if calibration samples with widely differing atomic numbers are used, then the measured X-ray attenuation values are subject to major errors. Further, the family of iso-absorption line curves that is determined does not allow exact definition of the density and atomic number functions for an X-ray apparatus.
In practice, this results in the difficulty that the density and atomic number functions calculated in the described manner do not reproduce the exact values of the density and atomic number of the calibration samples used to produce them. These discrepancies between the calculated and “measured values” and the actual and “nominal values” are also non-linearly dependent on the attenuation values μ1 and μ2, and cannot be handled by analytic methods. In many cases, this means that reliable characterization of a material or tissue is impossible.